R. Islam

Magnetic iron silicide nanowires on Si(110)

Self-assembled iron silicide nanowires were formed by depositing 1ML of Fe onto Si(110) at 700°C in ultrahigh vacuum. The nanowires have average dimensions of 5nm high ×10nm wide ×μm long, as measured with ex situ atomic force microscopy. High-resolution electron microscopy identifies the crystal structure as cubic FeSi2 with orientation FeSi2(1¯11)∕∕Si(11¯1), FeSi2⟨110⟩∕∕Si⟨110⟩. Magnetometer measurements show a magnetic moment of 0.3Bohr magneton per iron atom at 2K. This magnetic property in metastable cubic FeSi2 nanowires opens up the possibility for high-density data storage and logic applications.

Beneficial effects of annealing on amorphous Nb–Si thin-film thermometers

Amorphous Nb–Si alloys have a temperature-dependent resistivity which can be tuned over many decades by controlling composition and are used for thin-film thermometers. Annealing at temperatures from 100 to 500 °C produces dramatic but easily controlled increases in resistivity, both magnitude and temperature dependence, for insulating and metallic samples with compositions ranging from 8–15 at. %Nb. A transition from metal to insulator is induced by annealing an initially metallic sample. Annealing produces thermal stability against subsequent heat treatment, allowing annealed films to be used as low-temperature thermometers even when they are cycled to temperatures as high as 500 °C. Cross-section transmission electron microscopy and energy-dispersive x-ray analysis show that the initially amorphous films develop Nb-rich clusters within an amorphous Nb-depleted matrix, explaining the observed resistivity increase.

Effects of annealing on amorphous GdxSi1−x near the metal-insulator transition

Annealing of amorphous Gd–Si films produces large changes in magnetic and magnetotransport properties. The materials have spin-glass freezing and enormous negative magnetoresistance (MR) in the unannealed state but show drastically reduced MR and magnetization on annealing. These changes can be explained by high resolution transmission electron micrographs and energy-dispersive x-ray analysis which show the appearance of nanocrystalline clusters of GdSi and GdSi2 in an amorphous background. A comparison with the nonmagnetic analog Y–Si shows similar modification of electrical properties.

Concentration dependent microstructure and transport properties of the magnetic semiconductor Gd-Si

The transport properties and microstructure of amorphous GdxSi1−x alloys are presented. The conductivity increases from x=0 through the metal-insulator transition (x=14 at.%), up to a dopant concentration of 25 at.%. A sharp cusp in the magnitude of the conductivity is then observed and the flattening of the conductivity versus temperature curve occurs at higher concentrations. These transport results are explained in terms of high-resolution electron micrographs which demonstrate the formation of nano-crystallites at x≥25 at.%. The flattening of the conductivity versus the temperature curve is identical to the results for annealing of a-GdxSi1−x alloys with a low Gd concentration.